A solar cell element generally has a silicon-based, for example, p-type semiconductor substrate, an n-type diffusion layer, an antireflection film, aback surface electrode, and a front surface electrode. A conductive paste obtained by mixing conductive particles comprising silver as a main component with glass frit, an organic vehicle, a solvent, etc. is used when forming the front surface electrode. This conductive paste is formed into an electrode pattern by screen printing, stencil printing or the like, and then fired to form the front surface electrode.
In this firing process, normally the action of the glass frit contained in the conductive paste dissolves/eliminates the antireflection film. This establishes electrical contact between the front surface electrode and the diffusion layer. This step is generally referred to as “fire-through.”
In order for this fire-through process to be executed favorably, it is preferred that glass having good solubility with the antireflection film be used as the glass frit of the conductive paste. So far, glass containing lead oxide often has been used due to its excellent adherence to a semiconductor substrate and because it is easy to adjust its softening point and the fire-through process can be executed relatively favorably (see Patent Document 1, for example).
However, in the conventional glass containing lead oxide, fire-through is sometimes difficult to control, including cases of insufficient fire-through which causes a failure to achieve ohmic contact, or, excessive fire-through such that part of the front surface electrode intrudes deeply into the semiconductor substrate.
Although the use of glass that does not contain lead oxide was studied, it was still difficult to control the fire-through process, such as not being able to accomplish sufficient fire-through to obtain ohmic contact.